α₁-Adrenergic receptors: New insights and directions

The adrenergic receptors play a key role in the modulation of sympathetic nervous system activity as well as a site of action for many therapeutic agents. The α₁-adrenergic receptor subtypes (α_1A-, α_1B-, α_1D) are the prime mediators of smooth muscle contraction and hypertrophic growth, but their characterization in both binding and function have lagged the other adrenergic family members. Although they are derived from a related ancestral gene and all nine adrenergic receptor family members bind the endogenous ligands, epinephrine and norepinephrine, with roughly similar affinities, there are major differences in the mode of binding, second messenger utilization, and physiological effects of the α₁-subtypes compared with β-or α₂-subtypes. Here, we review the recent literature on aspects of its binding pocket and how it differs from the β-adrenergic receptor paradigms. We also review the signaling components and aspects of its function and provide new insights into its roles in smooth muscle, growth, neurological, and cardiovascular function.